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GLG345H1 Midterm: GLG345-ESS241 Midterm aid sheet

Earth Sciences
Course Code
James Mungall
Study Guide

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1. Introduction/Topographic Maps
- objective of structural geology? To understand the shape, size and orientation of geological bodies in the Earth, and their structures, with particular
emphasis on changes in these brought about by deformation.
- point on earth needs to be projected onto a plane, three main types: cylindrical, conical, planar
- every map distorts, some maps try to maintain: equal area, equal angle, equal distance (to one focal point), equal shape (conformal)
- stereographic preserves shape, orthographic looks realistic, azimuthal shows distance to centre point
- UTM (Universal Transverse Mercator) 60 horizontal cylinders touch the globe along a meridian covering 6 degrees each, northing = distance from
equator or south pole, easting = distance from central meridian plus 500km. - basis for topographical maps.
GPS - cartesian system, 24 satellites, each with precise atomic clock receiver compares time signal was sent to time signal was received, then calculates
distance and can triangulate location (and time) using at least 4 satellite signals.
- a geologic map is essentially an interpretation of the likely outcrop pattern of the rocks in an area, constrained by all the available data
- Strike and dip are essential measurements to take in the field. They are used to specify the orientation of a geological surface such as the interface
between two lithologies. The strike of a bedding is the direction of an imaginary horizontal line running along a planar bed. It is usually given as a
compass direction measured clockwise from the north. The angle of dip is the maximum inclination of the bed in degrees from the horizontal. The dip is
always measured perpendicular to the strike
2. Stratigraphy and Geologic Time
- Stratigraphy: description of bedded (sedimentary) rocks
- bed: has recognizable bottom and top, often related to one depositional event
- contact: surface separating beds (vertically), can be gradational or abrupt
- formation: distinctive (and mappable) series of strata, often named as place name + rock type (eg. Bright Angel Shale)
- relative time (order of events) vs. absolute time (when did it happen)
Stratigraphic principles:
1. sediments are deposited in horizontal layers
2. younger rock is deposited on top of older rock
3. young rock can contain pieces of an older rock
4. young rock or feature can cut across older rock or feature
5. young rock/feature can cause changes along their contacts with older rocks
- Unconformity: gaps in time, distinguished from other stratigraphic breaks by angular discordance between older beds below the unconformity surface
and younger beds above. deposition of lower strata - tilting or other deformation of lower strata - erosion - deposition of upper strata
- principle of fossil succession: can use fossils to determine relative time
1. Lithostratigraphy - describes the rocks, easier to work with. bed-member-formation-group - can determine climate with litho, different rocks in
different places
2. Biostratigraphy - looking at fossils, more linked to time, biozone. new species take time to spread out and then die, have to be selective with which
fossils we use
3. Chronostratigraphy - time succession, what we are really after, epoch-period-era-eon
3. Surface Processes and Landforms
- internal = endogenic: radiogenic heat, gravity, rock cycle and biogeochemical cycles
external = exogenic: solar energy, gravity, surface, biosphere
- river drainage basins, divides, patterns: dendritic, radial, structurally controlled
- profile of river determined by rock type, tectonics, climate, sea level = equilibrium profile. Erosion due to chemical/physical weathering, higher rates =
more sediment
- braided river: turbulent, large volume of bedload, banks erode easily, shifting bars
- meandering river: slow flow, suspended load, cohesive banks, cutbank, point bar, ox
- Mass movements: forces on any slope = friction and gravity. when gravity is stronger than friction, things move down a slope. mass movements are
classified according to the dominant material water or air content, and velocity of the movement
- Oceans: turbidities = graded bedding as top marker, flute marks, load casts
- Winds and Deserts: erosion and deposition by wind in arid climates, weathering = desert varnish, flash floods = dry wash, playas, wind = ventrifacts,
mesas, buttes
- ripples and dunes, deposition in a current, cross-bedding
- Glaciers: accumulation at high elevation, wastage at front, advance vs. retreat
- glacial till: unsorted, angular, ridges: major moraine, drumlins: show direction of glacier, scoured land when it moved, u-shaped valleys: river
cutting at bottom, sinking valley, arretes: jagged peaks stuck out of glacier, roche moutonee: flat side didn't erode but flattened, eskers: channels of
meltwater, glaciers retreated leaving river clogged
4. Stress and Strain
-Stress: (action) force over area (Pascal), what we want to find out. a pair of equal and opposite forces acting on unit area of a body.
-Strain: (response) response of body to stress, rock deformation, can be observed in the field. the geometrical expression of the amount of deformation
caused by the action of a system of stresses on a body.
- pressure or hydrostatic stress = dilation, compression or differential stress = contraction, tension or tensile stress = stretching, shear = shear
- stress on a plane decomposed into normal and shear component, shear stress largest at 45 angle but shear force largest at 90 angle (45 has largest
surface area)
- Mohr Circle: graphical representation of normal and shear stress acting on many possible planes through a point. radius=max shear, deviatoric=mean
to max/min, isotropic=mean
- strain can be translation, rotation, distortion (shape change), dilation (volume change)
- pure shear vs. simple shear, pure is coaxial, no rotation, simple has rotation
- Strain Markers: quantitative analysis eg. fossils broken apart, what are the directions of the principal strain axes? - originally perpendicular lines,
originally circular objects
- Stress Ellipsoid: stress acting on a point, consider tiny cube. stress components described by an ellipsoid
- Stress Tensor: also describes stress on a point, composed of 3 stress vectors defined in cartesian coordinates,
- stress in the lithosphere: free surface (earth's surface, tunnel wall) one of the principle stresses becomes zero, ellipsoid rotates, stress field also
deflected near a fault or fracture. - determining near-surface stress condition: from borehole breakouts, spalling in tunnel, earthquake data
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